The subject invention relates to a pre-processing apparatus for optimum overshoot control of an FM stereo system.
FM stereo radio broadcasting is a highly competitive medium. The need for loudness in FM has evolved as a result of this competitiveness and because FM is being used for broader markets such as country western, hard and soft rock, and other up-beat musical formats. Many FM stations are competing directly with AM stations and have developed a need for loudness.
An FM broadcaster must deal with modulation that is not only amplitude sensitive but frequency dependent as well. The maximum permissible deviation (100% modulation) is established by the FCC (Federal Communications Commission) as .+-.75 KHz. Further the FM transmitter includes a high frequency pre-emphasis network which has been standardized in the United States as one having a time constant of 75 microseconds. This pre-emphasis is such that a greater deviation at the higher frequencies is produced so as to overcome the noise rejection characteristics of the FM system which decreases at the higher frequencies. An FM receiver for use in the United States includes a 75 microsecond de-emphasis network. The standard 75 microsecond pre-emphasis presents a frequency dependent limit imposed by the severe high frequency boost given the audio signal. To prevent overmodulation or "overshoot" (exceeding .+-.75 KHz) FM limiters or clipping circuits have been utilized. Two basic approaches to the elimination of overmodulation caused by the 75 microsecond pre-emphasis have been used. One approach is a selective attenuation of the high frequency program content based upon the amount of energy present in these higher frequencies. In other words, the bandwidth of the audio is dynamically rolled off in direct proportion to the high frequency signal level. As the high frequency energy content increases, high frequency roll off occurs and levels at 100% modulation level. The frequency response is the inverse of the 75 microsecond pre-emphasis curve. This is an affective way of controlling overmodulation, but is obviously at the detriment of the response quality. A second approach is clipping. Implementing this concept requires only pre-emphasizing the audio and hard clipping at all the peaks at 100% modulation. Harmonic distortion generated by this process is attenuated by the subsequent de-emphasis in the receiver and potential out-of-band radiation is suppressed by the low-pass audio filters. Although this second approach has met with a greater acceptance, overshoots still occur. One of the reasons for this is related to the problem of D.C. restoration. Somewhere in the exciter, the audio signal which has been peak clipped is AC coupled. Whether it be the audio buffer, pre-emphasis amplifier or the 15 KHz low-pass filter the audio line eventually has a D.C. blocking capacitor. A slight asymmetry of audio signal that has been clipped causes a shifting of the entire signal at the D.C. blocking capacitor and the tightly held 100% peak is now overmodulating. This condition is largely uncorrected in previous peak clipping circuits since D.C. restoration usually involves a feed forward signal to input the proper polarity D.C. correction at the last AC coupled stage in the FM exciter of the transmitter. Broadcasters generally do not like to disturb the inner circuitry of the FM exciter.